1. Field of the Invention
The invention is directed to both a system and a technique for improved quantitative chemical analysis. More particularly, the invention provides for the use of a solid-state acousto-optic device in combination with a chromatographic column in order to provide a system for both the detection and the quantitative chemical analysis of a species of interest.
2. Description of the Prior Art
It is the conventional practice in the petrochemical industry, to utilize "on-line" sensors to affect chemical analysis. However, in such configurations, the presence of electrical or chemical sensors may pose a significant threat to the safety of the area, from either explosions or from chemical contamination. As a result, such "on-line" sensors must be contained in approved explosion-proof enclosures. These enclosures obviously represent a significant cost to the petrochemical industry.
It has been suggested by G. Schmidtke, et al. VDI-Berichte 509,293 (1984) that a fiber-optic system can be utilized to provide remote chemical analysis. This prior art system utilizes a diffraction grating and an array of detectors in a location remote from a sampling area.
The acousto-optic tunable filters (AOTF) has previously been used in spectral analysis as an effective device to measure dilute gas mixtures. An example of an automated AOTF infrared analyzer system which is usable in a variety of industrial and commercial control applications is disclosed in U.S. Pat. No. 4,490,845 to Steinbruegge et al., which patent is assigned to the assignee of the present invention and incorporated herein by reference as if fully set forth.
Concentrated mixtures of gases and especially liquids often have strong, nearly total optical absorption bands. To analyze these mixtures, one must often utilize the weaker overtone absorptions. The overtone bands of infrared absorptions lie in the near-to-intermediate infrared, where quartz fiber-optic attenuation is not prohibitive to the use of such fibers. Using optical fibers eliminates one of the constrains with present detection systems.
The quantitative chemical analysis of mixtures of gases or liquids is greatly simplified by the use of a chromatographic column. The column separates the mixture into its constituent parts by the process of differing diffusion rates of various species through the column. As the various specie emerge from the column they may be quantitatively measured by a variety of techniques, the measurement being greatly simplified by the separation of single emerging specie from others in the original mixture. The columns may be designed to separate various molecular species in liquids, in which case the process is termed "liquid chromatography", or to separate species in a gas mixture, which is termed "gas chromatography". Special variants of these are termed "HPLC, or, High Performance Liquid Chromatography", "Open column liquid chromatography", or "Super critical chromatography", etc.
As the individual specie emerge from the end of the chromatographic column they may be identified and quantified by a number of techniques, the technique chosen to be appropriate for the components of the mixture. Included among these are optical absorption (UV, visible, or IR), fluorescent excitation, refractive index, flame ionization, electrochemical or conductivity. The most commonly used techniques are by far the optical measurement techniques, especially UV absorption. Since the emergence of one specie from the column is followed shortly by another specie, measurements made "on line" at the column must be made quickly or interference between the species will occur and some error in the measurement will result. In many cases the measurement of detailed spectral information requires too much time to allow "on-line" measurements. In these cases the emerging specie is removed from the chromatograph and measured on a separate optical instrument. Much of the thrust in improving chromatographic instruments at the present time is devoted to improving the optical "on-line" detector so that as much information as possible can be obtained without requiring the removal of the specie sample.
A typical example of presently available technology for performing an optical absorption spectral dependence measurement is shown in Prior Art FIG. 12. Broadband light is produced by a discharge lamp "DL", passed through a flow cell "FC" containing the exiting specie to be measured. The spectral dependence of the light absorbed in the flow is analyzed by the combination of a holographic or diffraction grating "DG" and a photodiode array "PA". A lens system "LS" and shutter means "SM" are also illustrated. A similar arrangement would be used for analyzing the spectral dependence of the fluorescence emission of the sample, except that suitable wavelengths for exciting the fluorescence would be employed, and emitted fluorescent light would be analyzed. Additional examples are found in the following U.S. Pat. Nos. the contents of which are incorporated by reference as if fully set forth herein: 4,521,225 to Jenkins et al.; 4,541,269 to Thomas; 3,723,731 to Blau, Jr.; 3,995,960 to Fletcher et al. and 4,501,372 to Aine.
There are several disadvantages of this type of system. The photodiodes are much noisier than a photomultiplier tube detector and the diffraction efficiency of the grating, typically a holographic grating to obtain wide wavelength coverage, is at best 20%. What this means is that the signal/noise level on each detector element is low and the signals must be time averaged to obtain accurate data. The larger the number of diode elements used, the more serious this problem becomes, and the time required to obtain adequate signal/noise output can exceed the time between the arrival of the various species at the flow cell.
It is an object of the present invention to provide a system for chemical analysis which utilizes optical fibers to convey a light source to a sample and then from the sample toward a detector. This configuration would allow the use of such a remote chemical analysis system in applications where the presence of electrical or chemical sensors pose a significant threat from, for example, either explosions or from chemical contamination.
It is yet another object of this invention to provide an improved chemical analyzer which incorporates an automated acousto-optic system.
It is still another object of this invention to provide an improved chromatographic instrument in combination with an acousto-optic device.
It is another object of this invention to provide a method and an apparatus for determining the wavelength dependence of the fluorescence emitted by certain species when excited by light.
It is another object of this invention to determine the excitation spectrum of wavelengths that excite the fluorescence.